Multi-range electrical current measurement

1. An electrical current measurement system comprising:

a first low-range current gauge that receives a current to be measured and provides a first intermediate value according to a first measurement gain, the first intermediate value indicative of the amplitude of the current up to a first maximum current;

a second low-range current gauge that receives from the first low-range current gauge any portion of the current having a magnitude that exceeds the first maximum current and provides a second preliminary value according to a second measurement gain, the second preliminary value indicative of the amplitude of the portion of the current having a magnitude that exceeds the first maximum current up to a second maximum current; and

an arithmetic unit that receives the first intermediate value and the second preliminary value and provides a second intermediate value indicative of the amplitude of the current.

2. An electrical current measurement system as in claim 1 wherein the arithmetic unit multiplies the first intermediate value by a ratio of the second measurement gain to the first measurement gain to obtain a product, and adds the product to the second preliminary value to provide the second intermediate value.

3. An electrical current measurement system as in claim 1 and further comprising:

a full-range current gauge that receives the current and provides a full-range intermediate value indicative of the amplitude of the current; and

a selector that selects from among the first intermediate value, the second intermediate value, and the full-range intermediate value to provide a signal indicative of the amplitude of the current.

4. An electrical current measurement system as in claim 3 and further comprising:

a third low-range current gauge that receives from the second low-range current gauge any portion of the current having a magnitude that exceeds the second maximum current and provides a third preliminary value according to a third measurement gain, the third preliminary value indicative of the amplitude of the portion of the current having a magnitude that exceeds the second maximum current up to a third maximum current; and wherein:

the arithmetic unit receives the third preliminary value and multiplies the second intermediate value by a ratio of the third measurement gain to the second measurement gain to obtain a product, and adds the product to the third preliminary value to provide a third intermediate value, and

the selector selects from among the first, second, third and full-range intermediate values to provide a signal indicative of the amplitude of the current.

5. An electrical current measurement system as in claim 1 wherein the first low-range current gauge comprises:

a first trans-impedance unit that receives the electrical current to be measured; and

a first current bypass responsive to the first trans-impedance unit to divert any current having a magnitude that exceeds the first low-range maximum value from the first trans-impedance unit to the second low-range current gauge.

6. An electrical current measurement system as in claim 5 wherein the first trans-impedance unit comprises:

a first sense impedance;

a first sense amplifier in electrical communication with the first sense impedance and operative to provide the first intermediate value; and

a first composite amplifier in electrical communication with the first sense impedance.

7. An electrical current measurement system as in claim 6 wherein the first composite amplifier and the first sense impedance together define a frequency-compensated impedance lower than the sense impedance.

8. An electrical current measurement system as in claim 7 wherein the frequency-compensated impedance simulates an inductive load over at least a portion of its frequency range.

9. An electrical current measurement system as in claim 6 wherein the sense impedance comprises a resistor and a capacitor in a parallel configuration.

10. An electrical current measurement system as in claim 6 wherein the first sense amplifier comprises an instrumentation amplifier.

11. An electrical current measurement system as in claim 5 and further comprising a bypass capacitor in parallel with the first current bypass.

12. An electrical current measurement system comprising:

a first low-range current gauge that receives a current to be measured and provides a first intermediate value according to a first measurement gain, the first intermediate value indicative of the amplitude of the current up to a first maximum current;

a second low-range current gauge that receives from the first low-range current gauge any portion of the current having a magnitude that exceeds the first maximum current and provides a second preliminary value according to a second measurement gain, the second preliminary value indicative of the amplitude of the portion of the current having a magnitude that exceeds the first maximum current up to a second maximum current; and

a processor that receives the first intermediate value and the second preliminary value and provides a signal indicative of the amplitude of the current.

13. An electrical current measurement system as in claim 12 and further comprising

a third low-range current gauge that receives from the second low-range current gauge any portion of the current having a magnitude that exceeds the second maximum current and provides a third preliminary value according to a third measurement gain, the third preliminary value indicative of the amplitude of the portion of the current having a magnitude that exceeds the second maximum current up to a third maximum current; and

a full-range current gauge that receives the current and provides a full-range intermediate value indicative of the amplitude of the current.

14. An electrical current measurement system as in claim 13 and further comprising a storage unit and instructions in the storage unit operative to cause the processor to:

multiply the first intermediate value by a ratio of the second measurement gain to the first measurement gain to obtain a product and add the product to the second preliminary value to obtain a second intermediate value;

multiply the second preliminary value by a ratio of the third measurement gain to the second measurement gain to obtain a product and add the product to the third preliminary value to obtain a third intermediate value;

select the first intermediate value if that value is indicative of a current magnitude less than the first maximum current, else to select the second intermediate value if that value is indicative of a current magnitude less than the second maximum current, else to select the third intermediate value if that value is indicative of a current magnitude less than the third maximum current, else to select the full-range intermediate value; and

provide a signal indicative of the amplitude of the current according to the selected value.

15. An electrical current measurement system as in claim 14 and further comprising a visual display that receives the signal and provides a visual indication of the amplitude of the current.

16. A method of measuring an electrical current comprising:

measuring that portion of an electrical current having a magnitude that does not exceed a first low-range maximum current to obtain a first intermediate value;

measuring that portion of the current having a magnitude that exceeds the first low-range maximum current to obtain a second preliminary value; and

calculating a second intermediate value indicative of the current from the first intermediate value and the second preliminary value.

17. A method as in claim 16 wherein calculating the second intermediate value comprises:

multiplying the first intermediate value by a ratio of a second low-range measurement gain to a first low-range measurement gain to obtain a product; and

adding the product to the second preliminary value to provide the second intermediate value.

18. A method as in claim 17 and further comprising:

measuring that portion of the current having a magnitude that exceeds a second low-range maximum current to obtain a third preliminary value;

multiplying the second intermediate value by a ratio of a third low-range measurement gain to the second low-range measurement gain to obtain a product; and

adding the product to the third preliminary value to provide a third intermediate value.

19. A method as in claim 18 and further comprising directly measuring the current to provide a full-range intermediate value.

20. A method as in claim 19 and further comprising:

selecting from among the first, second, third and full-range intermediate values; and

providing a signal indicative of the amplitude of the current according to the selected intermediate value.